The present invention relates to a tracking device for use in a helical scan type magnetic recording and reproducing apparatus, etc., and particularly to a tracking device of an ATF (Automatic Track Finding) system which uses four-frequency pilot signals recorded sequentially on helical tracks of a magnetic tape.
Prior art references related to such a tracking device are, for example, JP-A-59-36358, JD-A-59-68862 and JP-A-5975450.
A construction of a conventional tracking device of the ATF system is shown in FIG. 23, which corresponds to S. Itoh et al., "MULTI-TRACK PCM AUDIO UTILIZING 8 mm VIDEO SYSTEM", IEEE Transactions on Consumer Electronics, Vol. CE-31, No. 3, August 1985.
Before describing the conventional tracking device in detail, the four-frequency pilot signals will be described first.
As shown in FIG. 23, four-frequency pilot signals having frequencies f1 to F4 are recorded sequentially on respective tracks of a magnetic tape 1 together with information. These pilot signals are obtained by dividing an oscillator frequency of 378fH from a clock generator 15 by 58, 50, 36 and 40, respectively, by means of a clock frequency divider 17 and thus fi.apprxeq. 6.5 fH, f2.apprxeq.7.5 fH, f3.apprxeq.0.5fH and f4.apprxeq.9.5 fH, where fH is a horizontal sync signal frequency of a television signal and 1fH=15.73 kHz. Therefore, a difference in pilot signal frequency between adjacent tracks on the magnetic tape 1 is always substantially 1fH or 3fH, although it is actually 16.407 kHz (or 16.52 kHz) or 46.145 kHz (or 46.209 kHz), respectively.
Assuming that a magnetic head 2 is scanning a track recorded with the pilot signal frequency f2, the frequency difference between the pilot signal f1 on a preceding track and the pilot signal f2 is 1fH and that between the pilot signal f2 and a pilot signal f3 on a succeeding track is 3fH.
In FIG. 23, the magnetic tape 1 is transported by a capstan 22 driven by a capstan motor 23 which, in turn, is driven by a motor driver 24. Information recorded on the magnetic raise 1 is picked up by the magnetic head 2 and amplified suitably by a pre-amplifier 3. Thereafter, it is filtered by a LPF (Low Pass Filter) 4 to remove high frequency video or audio information components which are unnecessary for tracking control. Then, the reproduced pilot signals are amplified again to a suitable level by an AGC (Automatic Gain Control) amplifier 5 and supplied to a balanced modulator 28 which is a multiplier.
The balanced modulator 28 multiplies the reproduced pilot signal with a local pilot signal supplied from the clock frequency divider 17 to convert the pilot signals of both adjacent tracks into signals of 1fH and 3fH. When the magnetic head 2 scans the track recorded with the pilot signal f2, the clock frequency divider 17 provides a local pilot signal of f2 according to a control signal SEL supplied from an input terminal 19.
In this example, the reproduced pilot signals contain the pilot signal f2 on the track under scan and frequencies f1 and f3 of the pilot signals on the two tracks adjacent thereto. Therefore, an output of a multiplier 28 contains frequency components f2+f1 and f3+f2. The pilot signal f2 of the track under scan becomes a zero beat due to this multiplication.
The output of the multiplier 28 is supplied to 1fH-band pass filter (BPF) 8a and 3fH-BPF 9a from which frequency components f2-f1.apprxeq.1fH and f3-f2.apprxeq.3fH are derived, respectively. As to the 1fH and 3fH signals, the 1fH signal is obtained by frequency-conversion of the pilot signal f1 of the preceding track and the 3fH signal is obtained by frequency-conversion of the f3 pilot signal of the succeeding track. Therefore, by comparing the level of the fH signal with the level of the 3fH signal, it is possible to detect a position of the track which is currently being scanned by the magnetic head 2, that is, the tracking state.
The 1fH signal and the 3fH signal derived from the 1fH-BPF 8a and the 3fH-BPF 9a are supplied to a peak detector 11a or 12a through a switch 13a. The switch 13a is switched at a track scan period by a control signal HSW supplied from an input terminal 20. A variation of a relation between the pilot signals of the preceding and succeeding tracks and the frequency converted lfH and 3fH signals varies every track. That is, for example, in the state shown in FIG. 23, the pilot signal of the preceding track is converted into the 1fH signal and the pilot signal of the succeeding track is converted into the 3fH signal. However, when the magnetic head 2 is scanning the track recorded with the pilot signal f3, the local pilot signal supplied to the multiplier 28 is f3 and therefore the pilot signal f2 reproduced from the preceding track becomes f3-f2.apprxeq.3fH and the pilot signal f4 reproduced from the succeeding track becomes f4-f2.apprxeq.1fH Therefore, the relation between the pilot signals of the preceding and the succeeding tracks and the frequency converted lfH and 3fH signals becomes reversed compared with the case shown in FIG. 23. The switch 13a functions to cancel out such variation of the relation between the pilot signals of the preceding and succeeding tracks and the 1fH and 3fH signals.
The 1fH and 3fH signals switched at the track scan period are supplied to the peak detector 11a or 12a. The peak detectors 11a and 12a detect peak levels of the 1fH and 3fH signals, respectively, and supply them to a subtracter 14a. The subtracter 14a provides a difference therebetween, that is, a tracking error signal, and supplies it to an adder 27. The adder 27 adds it to a speed error signal supplied from a capstan speed control circuit 26 and a result is supplied to the motor driver 24. The motor driver 24 supplies electric mower corresponding to the sum of the tracking error signal and the speed error signal to the capstan motor 23 to drive the latter. Thus, the capstan 22 transports the magnetic tape 1 at a predetermined speed in a predetermined phase.
The capstan speed control circuit 26 measures a period of a CFG signal produced proportionally to rotation of the capstan 22 and outputs a difference between the measured period and a desired period as the speed error signal to be supplied to adder 27.
The clock frequency divider 17 for producing the local pilot signals divides a clock supplied from the source clock generator 15 which includes a stable quartz oscillator 16 and produces the local pilot signals f1 to f4 sequentially at the track scan period according to the control signal SEL. The AGC amplifier 5 functions to make the reproduced pilot signal level constant such that the tracking error signal does not substantially vary due to variation of the level of the reproduced pilot signals.
In FIG. 23, signals to be processed by the multiplier 28 for frequency-converting the reproduced pilot signals, the BPFs 8a and 9a for deriving the lfH and 3fH signal components, the peak detectors 11a and 12a and the comparator 14a for comparing the 1fH and 3fH signals are analog signals. Therefore, such an ATF analog tracking control system can not be combined easily with a tape speed control system and a speed and phase control system for a drum on which magnetic heads are mounted, both of which are currently digital or software-implemented. In other words, since the ATF tracking control system operates in an analog manner, it is very difficult to highly integrate the ATF tracking control system with other digital control systems. Further, the characteristics of the BPFs for deriving the 1fH and 3fH signals which affect the performance of the ATF tracking control system substantially may be degraded due to variation of constitutional Darts and their time-dependent changes when they operate in analog manner.